1. Field of the Invention
The present invention relates to a stator core for a rotating electrical machine and a method of manufacturing the same.
2. Description of the Related Art
As a stator core for rotating electrical machines, a so-called “helical stator core” has been used heretofore. As shown, for example, in Japanese Patent Laid-open Publication (JP-A) No. 2001-054261, the helical stator core (hereinafter referred to for brevity as “stator core”) is formed by helically winding a strip of steel sheet press-formed with magnetic pole teeth while, at the same time, stacking the helically wound portions or turns into a cylindrical shape. An outer edge portion of the strip, which forms a core-back part of the stator core, is thinned into a tapered shape and extended in the longitudinal direction by a pair of forming rollers to provide a longer circumferential length for facilitating winding of the strip.
The helically wound portions or turns of the strip are brought together by axial compaction to thereby form a laminated stator core. The turns or layers of the laminated stator core are aligned in the radial direction such that an amount of radial displacement between the layers is within an allowable range. The thus aligned layers of the laminated stator are welded at respective outer edges thereof to thereby keep the interlayer displacement from becoming greater. The laminated stator core is then subjected to a finishing process in which, as shown for example in Japanese Patent Laid-open Publication (JP-A) No. 2006-246586, the laminated stator core is shaped by ironing into a desired form having a desired level of dimensional accuracy in terms of the roundness of an outer peripheral surface, concentricity of the outer peripheral surface relative to an inner peripheral surface, and perpendicularity of the outer peripheral sure.
According to the conventional winding process described above, the steel sheet strip having press-formed magnetic pole teeth is bent edgewise into an arc with a toothed edge of the strip being radially inside and a core-back side edge opposite the toothed edge being radially outside. During such edgewise bending, an outer edge portion of the core-back side edge is thinned into a tapered form and extended in the longitudinal direction. Since the core-back side becomes thinner as it approaches the outer edge thereof, a crack may occur at thus thinned or tapered core-back side.
To deal with this problem, a separate pre-winding process is employed in which the core-back side of a steel sheet strip is thinned and extended in the longitudinal direction by means of forming rollers to provide an increased circumferential length for winding and cause a plastic flow of the material in a feed direction of the strip during rolling with the result that the strip bends edgewise in a width direction to assume a bent shape, which can facilitate a subsequent helical winding operation.
However, as for the thinning by the forming rollers and the edgewise bending, the current tendency is to use a thinner steel sheet so as to lower eddy-current loss (iron loss) while allowing widening of a core-back portion to deal with a high-power trend of the rotating electrical machines in recent years. Thus, the thinning work necessarily becomes heavy-load work and requires a large working machine making it more difficult to meet power saving demands. Furthermore, excessive thickness reduction may cause a core-back cracking problem dug the winding process.
As the thickness reduction of core-back part increases, a wedge-shaped gap or clearance produced between the outer edge portions of the thinned core-back parts of two adjacent layers of the laminated core becomes large. Due to the presence of a large wedge-shaped interlayer clearance, the ironing process achieved for shaping an outer peripheral surface of the core into a desired form will encounter a problem that the outer edge portions of the adjacent layers of the core cannot be securely restrained despite being applied with a proper ironing load or pressure for an appropriate ironing depth and tend to weaken or absorb the ironing pressure via the interlayer clearance. In the case where the outer edge portions of the layers are largely displaced from one another in the radial direction, the ironing operation becomes sluggish and causes distortion of the outer edge portions, leading to enlargement of the interlayer clearance.
The enlarged interlayer clearance and the crack at the core-back part, which are caused by intensified thinning of the core-back part, deteriorate the aesthetic appearance of the core, will cause rust on the core, and increase magnetic resistance and hinder smooth passage of magnetic fluxes and thereby lower the output power of the rotating electrical machine. Furthermore, the interlayer clearance might lower a clamp-holding force by deflecting an axial component thereof when the stator core and front and rear frames are clamped in sandwiched relation by means of through-bolts.
To cope with the growing tendency to increase the power and efficiency of the rotating electrical machines, the stator core needs to be larger in size and diameter and able to accommodate conductors within slots at a higher density. To meet these requirements, the thickness of a steel sheet should be reduced to thereby ensure that the stator core has an increased number of slots, a smaller slot pitch, a larger core-back part, and a lower iron loss.
As described above, the use of a thin steel sheet would cause various problems in conjunction with the winding process during the manufacture of a cylindrical stator core, such as a difficulty in obtaining a large rolling reduction when an outer edge portion of the core-back part is thinned by rolling, an enlarged interlayer clearance caused at the outer edge portion of the core due to an increase in the rolling reduction, and a crack occurring at the thinned or rolled core-back part of the core. Furthermore, the ironing process also encounters difficulties that smooth ironing work cannot be achieved and the interlayer clearance is enlarged by the ironing process.